Device-to-device (D2D) communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite, such as WiFi Direct. These systems operate in unlicensed spectrum. The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) specifications refer to D2D communications as sidelink communications.
D2D features include both commercial and public safety applications. Some applications include device discovery, where devices are able to sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application involves direct communication based on physical channels terminated directly between devices.
Another D2D application includes V2x communication, which refers to any combination of direct communication between vehicles, pedestrians and infrastructure. V2x communication may take advantage of a network infrastructure, when available. Even when out-of-coverage, basic V2x connectivity is possible. An LTE-based V2x interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the network infrastructure (V2I) and vehicle-to-person (V2P) and vehicle-to-vehicle (V2V) communications, as compared to a dedicated V2x technology.
FIG. 1 is a block diagram illustrating an example LTE V2x network. Wireless devices 210 may communicate with a network node 220 using wireless signals 230. Wireless devices 210 may communicate with other wireless devices 210 using wireless signals 240. Wireless devices 210 may include handsets and vehicles, such as passenger cars, trucks, and busses equipped with wireless transceivers.
V2x communications may include both safety and non-safety information. Each type of information may be associated with applications and services that have specific requirements, for example, in terms of latency, reliability, capacity, etc.
ETSI has defined two types of messages for road safety. The two types are Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM).
CAM messages enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters via a broadcast. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. CAM messages also provide active driving safety assistance for normal, non-emergency traffic. A wireless device may check for the availability of a CAM message every 100 ms, yielding a maximum detection latency requirement of less than or equal to 100 ms for most messages. The latency requirement for a pre-crash sensing warning, however, is 50 ms.
DENM messages are event-triggered, such as by braking. A wireless device may also check for the availability of a DENM message every 100 ms. The maximum latency requirement is less than or equal to 100 ms.
The size of CAM and DENM messages may vary from 100+ to 800+ bytes. A typical size is around 300 bytes. The messages are supposed to be detected by all vehicles in proximity. The Society of the Automotive Engineers (SAE) also defines a standard message referred to as the Basic Safety Message (BSM) for dedicated short-range communications (DSRC). BSM defines various message sizes. BSMs may be classified into different priorities based on the importance and urgency of the messages.
When a D2D interface is used for direct communication between wireless devices, particularly for vehicular applications, multiple transmission formats, resource allocation methods, and transmission periodicities need to be accommodated on the same carrier. One way to provide this, according to the principles of LTE D2D in Rel-12, is to define multiple pools each associated to specific transmission format and resource allocation parameters. A problem with this solution, however, is that such pools may use orthogonal radio resources to reduce interference, which causes serious resource fragmentation for the carrier. This degrades radio utilization efficiency.